An inorganic electroluminescence (EL) light-emitting device (body) is a light-emitting device in which an alternating current electric field is applied to an inorganic phosphor particle sandwiched between two electrodes, to cause the phosphor to emit light. Known examples of the EL light-emitting device include dispersion-type EL light-emitting devices and thin-film-type EL light-emitting devices. The dispersion-type EL light-emitting device has a structure where a phosphor particle with a diameter of several μm dispersed in a binder with high dielectric constant are sandwiched between two electrodes at least one of which is transparent, is a plane light-emitting device allowing the light-emitting device to have a thickness of several mm or less, and has many advantages in that, e.g., there is no exothermic heat and a luminescent efficiency is favorable. Therefore, the dispersion-type EL light-emitting devices are expected to have many applications for traffic sings, lighting equipments for various interiors and exteriors, light sources for flat panel displays such as liquid-crystalline displays, light sources of lighting equipments for large-area advertising pillars, and the like.
As an EL phosphor particle for use in the inorganic EL light-emitting device, an inorganic semiconductor particle to which an activator (metal ion serving as a luminescence center) is added, is used. An EL phosphor particle well known comprises zinc sulfide as a host material thereof, along with an activator such as copper and a co-activator such as chlorine added thereto. However, the light-emitting device produced by using the phosphor particle has some drawbacks in that its luminance is low and its light emission life is short, as compared with those of light-emitting devices based on any other principle, and therefore, various improvements have heretofore been made on the phosphor particle.
In a typical method of synthesizing the phosphor particle including zinc sulfide as the host material, a zinc sulfide nano particle which is a raw material is subjected to a first sintering (baking) in combination with an inorganic salt, called a flux, at extremely high temperature of 1,300° C. to 1,000° C., to grow the particle with a micron size; and then a second sintering is performed at 500° C. to 1,000° C., to yield phosphor particle. This producing method is described in, for example, JP-A-8-183954 (“JP-A” means unexamined published Japanese patent application), JP-A-7-62342, and JP-A-6-330035. However, in this method, the sinterings are performed in a furnace at high temperature, making it difficult to add materials to the system from the start to the end of the sinterings. For example, it is impossible to change concentration distribution of the activator or the co-activator inside the particle. Thus, it is difficult to obtain a zinc sulfide phosphor particle with higher luminance.
Meanwhile, when the zinc sulfide particle is synthesized in a liquid phase, it is possible to add the activator or co-activator with its amount controlled during growth of the particle. Thus, it is possible to produce a phosphor particle in which the concentration distribution of the activator or co-activator inside the particle is changed, which cannot be obtained by the sintering method. Also, it becomes possible to obtain a zinc sulfide particle with monodisperse where a size distribution is narrow, by forming the particle in the condition that nucleus formation is separated from the growth, and controlling a supersaturation degree during growth of the particle.
As the method of synthesizing a zinc sulfide particle in the liquid phase used for the inorganic EL phosphor, there are a method of synthesizing a particle with a nano size in an aqueous system as disclosed in JP-A-2002-313568, and reports in which a crystal of zinc sulfide is grown up to submicron size in an aqueous system in “Fine Particles” (Surfactant Science Series, Volume 92, edited by Sugimoto, pp. 190-196), “Colloids and Surface A” (Vol. 135, pp. 207-226 (1998)), and “Crystal Research Technology” (Vol. 35, pp. 279-289 (2000)). However, a spherical particle obtained by the method described in “Fine Particles” is a secondary particle (aggregate of particles) formed by aggregating microcrystals with a nano size, and a particle obtained by the method described in “Colloids and Surface A” is also an aggregate of particles with small size. In these methods, a primary particle with a micron size which is desirable for the EL phosphor particle can not been obtained. Meanwhile, according to the method described in “Crystal Research Technology”, a zinc sulfide primary particle with a submicron size can been obtained by placing a particle with a nano size previously prepared in a sealed vessel, and maturing the particle at a high temperature. However, it is difficult to modify the particle during growth of the particle in this method, and it is impossible to control the concentration distribution of an activator and co-activator inside the particle and the particle size distribution.